This invention relates to an electronic safety system for the avoidance of an overspeed condition in the event of a shaft failure by detection of the shaft failure and subsequent interruption of the further energy supply, in particular on a gas turbine engine.
In the event of a failure of a shaft, which has a driving side and a driven side, i.e. an energy-generating and an energy-consuming side, the driving side which is now detached from the energy-extracting driven side will, as a general problem, be accelerated considerably, this resulting in a serious hazard to persons and material. Such an overspeed condition is particularly problematic where the respective shaft is part of a means of passenger transport, for example an aircraft powered by gas-turbine engines. In an aircraft engine, the failure of the low-pressure turbine shaft, in particular, and the resultant uncontrolled speed increase of the driving side of the low-pressure turbine shaft connected to the low-pressure turbine rotor can lead to the destruction of the engine and damage of the aircraft, thus constituting a considerable danger to persons and property.
In particular on gas turbines, especially gas turbine engines, various devices for the mechanical or electronic detection of a shaft failure and for the subsequent interruption of the further fuel supply to avoid or control a dangerous overspeed condition are known.
In a safety system described in Patent Specification U.S. Pat. No. 4,712,372, inductive sensors are arranged on both the toothed driving side (turbine rotor) and the toothed driven side (fan) of the turbine shaft which produce a speed-proportional signal corresponding to the number of pulses counted. If a speed difference resulting from a higher speed of the driving-side end of the shaft and, thus, a shaft failure is registered, a fuel solenoid valve is actuated and the fuel supply interrupted in order to stop powering the turbine rotor.
Generally, the known electronic safety systems of gas turbine engines are disadvantageous in that the time period until shut-off of the fuel supply is relatively long, this resulting in increased strength requirements on the low-pressure turbine blades in connection with a higher weight and higher costs. High investment is also incurred by the required cooling or heat shielding of the sensors and the electric connections situated in the hot zone of the low-pressure turbine shaft.
Furthermore, mechanical shut-off systems are described in which a reference shaft is coaxially associated to the turbine shaft and connected to the driven end of the turbine shaft. In the event of a shaft failure, the resultant rotation of the turbine shaft relative to the reference shaft is used to mechanically actuate the fuel valve. In a known device of this type, offset recesses are provided on the driving end of the turbine shaft and on the corresponding end of the reference shaft, which, in the case of a shaft failure, will come into coincidence, thereby releasing a pre-loaded driver. The driver, which swings out radially, engages a loop of a wire rope which is connected to the fuel shut-off valve and which closes the fuel-shut off valve by the pull exerted on it by the driver of the low pressure shaft.
Since the required angle of rotation between the turbine shaft and the reference shaft until release of the pre-loaded driver is relatively large, the time until shut-off of the fuel supply will be correspondingly long. Also, the mechanical systems make great demands in terms of design, assembly and space.